Numerical modeling on post-local buckling behavior of circular and square concrete-filled steel tubular beam columns

In concrete-filled steel tubular (CFST) beam-columns under cyclic loadings, local buckling of restrained steel tube, crushing and slipping of the confined concrete could have significant influences on strength and stiffness degradation. This paper presents a numerical model based on fiber element discretization for simulating the deterioration behaviour of both rectangular and circular CFST beam columns under axial load and cyclic loading. Uniaxial stress-strain relationships for steel and concrete in CFST beam columns are proposed by introducing unified parameters to control the extent of deterioration. Axial contraction caused by local buckling was accounted by defining an inelastic region that promises plasticity and strength degradation. The numerical model was validated by comparing the simulation results and the associated experimental results. The influences of diameter-to-thickness ratio, concrete and steel strength ranging from normal strength to high strength on the deterioration behavior of circular and rectangular CFST beam-columns were analyzed. Finally, through recognizing the limit state to trigger global strength degradation of a CFST frame, the developed numerical model is applicable for simulating global behavior of CFST frames under collapse-level deformations.